The energy for operation of such a transponder in the receive and transmit modes is obtained from an interrogator transmitter, by rectifying a received interrogation RF signal. The energy is stored in a charge capacitor. In a HDX (Half Duplex) CMOS integrated transponder circuit, the inherent substrate diode (formed by any of the n-diffusion areas connected to the antenna RF terminal in conjunction with a semiconductor p-substrate), is used as a rectifier diode. With such a rectifier diode, only the negative half-wave of the received RF signal is used, and the resulting DC voltage stored in the charge capacitor is limited to the peak voltage of the RF signal.
For applications that require a relatively large range of the transponder's transmitter, a higher supply voltage is required than can be obtained with a half-wave rectifier. A voltage twice that which can be obtained with a half-wave rectifier would be obtained with a full-wave rectifier.
A full-wave rectifier would require a second rectifier diode in addition to the existing substrate diode. Incorporation of an additional diode within the substrate would require a well process with an isolated p-well area. Available CMOS processes for low-cost production of transponder devices do not have this option.
A high voltage PMOS transistor that could be connected as a diode is also not available with this technology.
A second rectifying diode could be formed by a diode-connected NMOS transistor. However, the required n-diffusions together with the p-substrate inevitably create a parasitic npn-structure that behaves as a bipolar transistor. The parasitic npn-structure destroys the reverse isolation of the rectifier diode formed by the NMOS transistor during the negative half wave when the substrate diode is conducting.